Well, it's a slim chance that this is what's different but my test system is already running 6.10 from the CR repo. If you want to try that then yum install centos-release-cr then yum --enablerepo=cr update (might need --noplugins if you have priorities assigned at the moment).

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So now I get /dev/pts/1 on my connect ppp0 line. I don't know for sure if not running modprobe pppol2tp is the difference yet, but it seems quite likely. But I'm still not connecting successfully.

I have been setting up wpa_supplicant in preparation of rebooting with NetworkManager disabled. Not likely, but I wondered if NetworkManager is interfering with xl2tpd. At least I have learned how to set up wireless networks without NetworkManager if nothing else.